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Sea Ice

The U.S. Coast Guard icebreaker Healy moves through pancake ice in the Arctic's Chukchi Sea.

The U.S. Coast Guard icebreaker Healy moves through pancake ice in the Arctic’s Chukchi Sea. (Photo by Carin Ashjian, Woods Hole Oceanographic Institution)

What is Sea Ice?

Sea ice is frozen seawater floating on the surface of the ocean. Unlike icebergs, which originate from land-based sources like glaciers and ice sheets, sea ice is formed entirely in the ocean.

Sea ice exists in the Arctic Ocean and the far northern and southern reaches of the Pacific, Atlantic, and Southern Oceans. It forms and melts each year with the seasons under the influence of several factors, including air and water temperature, the amount of sunlight reaching the ocean surface and the age of the ice.

During the Northern Hemisphere winter, the extent of Arctic sea ice increases and generally reaches its maximum in March. As the days lengthen, sea ice extent gradually shrinks until reaching its minimum sometime in September. First-year ice—ice that has only existed for one winter season—generally melts more easily than older, multi-year ice because it is often thinner.

Warming in the Arctic is causing the annual extent of sea ice to shrink each winter. This is also shrinking the percentage of multi-year ice in the ice pack, which leaves it susceptible to further reductions each summer.


Why is it important?

Sea ice plays an important role in Earth’s climate system. Because it is white, it reflects as much as 90 percent of the sunlight that lands on it. As the extent of sea ice in the Northern and Southern Hemispheres shrinks, it is exposing more of the ocean beneath. Because it is darker than sea ice, the ocean naturally absorbs more of the sunlight that lands on it, warming the water and causing additional melting to occur.

Sea ice is also an important part of the Arctic and Antarctic ecosystems. The bottom and interior of sea ice is riddled with brine-filled channels that serve as a home for bacteria, viruses, unicellular algae, diatoms, ice worms and small crustaceans. As the ice warms in spring and summer, these tiny organisms are released into the surface water, where they become food for a wide range of fish and shrimp, which in turn become food for larger animals.

In the winter, these sympagic (ice-reliant) organisms continue to survive and to thrive, providing a possible model for scientists interested in understanding how life could exist inside or beneath the frozen surface on some moons of Jupiter and Saturn.

Terrestrial animals, such as penguins in the Antarctic and polar bears in the Arctic, also rely on sea ice to provide a platform on which to hunt and rest.

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News & Insights

Imagining Home: scientist’s stay in the Arctic extended by coronavirus

WHOI biologist Carin Ashjian is aboard the icebreaker Polarstern in the Arctic as part of the year-long MOSAiC research expedition. She should be almost home by now. Instead, her stay has been extended by COVID19.

A sea of ancient ice

WHOI scientist dusts off historical accounts to tackle the long-standing mystery of just how thick Arctic sea ice was in the early 19th century.

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News Releases

Epic Arctic Mission Ends

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WHOI in the News

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From Oceanus Magazine

Communicating Under Sea Ice

Researchers at Woods Hole Oceanographic Institution developed a new communication and navigation system that works over long distances under Arctic sea ice, allowing scientists to use autonomous underwater vehicles to…

A Buoy’s Long Strange Trip

Since 2004, WHOI scientists have deployed ice-tether profilers (ITPs) in polar sea ice to monitor changing conditions in the Arctic. ITP 47 found its way to the coast of Ireland.

Another Piece in the Arctic Puzzle

It’s spring again, and while most of us are putting away our winter coats and watching our flowers pop up, it’s time for Rick Krishfield and Kris Newhall to don their extreme weather gear, zip up to the top of the world, and plant scientific instruments on an ice floe in the Arctic Ocean. Their…

Exploring the Arctic in the Midst of Change

Chief Scientist Bob Pickart and his 26-member science team were in the hangar at the Barrow Air Search and Rescue Station, waiting for the helicopter. An Inupiat community barnacled to a rocky beach at the northern tip of the North Slope of Alaska, Barrow is the most practical departure point for research cruises into the…

The Icebot